Method of forming a transparent reflection reducing coating on glass and the articleresulting therefrom



May 26, 1953 s, McLEAN 2,639,999

METHOD OF FORMING A TRANSPARENT REFLECTION REDUCING COATING ON GLASS AND THE ARTICLE RESULTING THEREFRQM Filed 001;. 29, 1949 lnven rofi: S tuawt McLean His A t bcgvnes.

Patented May 26, 1953 METHOD OF FORIVHNG A TRANSPARENT RE- FLECTION REDUCING COATING ON GLASS AND THE ARTICLE RESULTING THERE- FROM Stuart McLean, Willoughby Township, Lake County, Ohio, assignor to General Electric Company, a corporation of New York Application October 29, 1949, Serial No. 124,449

8 Claims.

My invention relates to reflection-reducing coatings and articles provided with such coatings, and to a method of producing such coated articles. The present application is a continuation-in-part of my co-pending U. S. application Serial No. 105,535 filed July 19, 1949 and assigned to the assignee of the present application, and

now abandoned.

It is well known to apply coatings of various nature to the surface or surfaces of an article for the purpose of reducing the surface reflections from the article with substantially no light absorption. Thus, the presence on the surface of a glass article of a coating of colloidal silica having an optical thickness of the order of, or an odd multiple of, one-fourth the wave length of the light transmitted will substantially eliminate the surface reflections from the glass article and increase the percentage of incident light transmitted through the article.

While these colloidal silica coatings are entirely satisfactory insofar as their reflection-reducing properties are concerned, their permanency or durability heretofore has been inadequate for certain applications. Thus, where such colloidal silica reflection-reducing coatings are applied to the outer surface of the lens of a vehicle headlamp which is subject to weathering, wiping, handling, and the like, the coatings do not remain on the article for as long a period as is desired. Usually, such coatings will wash or rub off the first time the headlamp is washed or wiped.

It is an object of my invention, therefore, to provide a substantially transparent reflection-reducing surface coating of the above-described character which is highly durable and permanent, and will withstand a reasonable amount of weathering, washing, wiping and cleaning.

Another object of my invention is to provide an article having a substantially non-reflecting surface of highly durable and permanent character.

Still another object of my invention is to provide an improved method of applying a reflection-reducing coating to the surface of an article whereby the coating is rendered more resistant to removal. V

In accordance with the invention, the durability or permanence of reflection-reducing coatings is increased to a considerable degree by the incorporation in the coating medium, prior to the application thereof to the article to be coated, of a suitable amount of a substance which under the action of heat to the proper degree will act in the manner of a flux to stick the particles comprising the reflection-reducing coating to the surface of the article.

Further objects and advantages of my invention will appear from the following detailed description of a species thereof and from the accompanying drawing.

In the drawing, Fig. 1 is an elevation partly in section of an electric incandescent lamp of the reflector type provided with a reflection-reducing coating according to the invention, and Fig. 2 is a greatly enlarged fragmentary sectional view of the lens of the lamp shown in Fig. 1.

While the invention is applicable in general to any article which is of a light-transmitting character, such asglass lenses, plates, prisms, or other optical elements and similar elements formed of plastic or artificialresin, it is herein illustrated as applied to the cover glass or lens of a reflectortype electric incandescent lamp of the wellknown sealed beam form described and claimed in U. S. Patent 2,148,314, D. K. Wright, issued February 21, 1939, and assigned to the assignee of the present invention.

As shown in Fig. 1, the said lamp comprises a preformed glass lens or cover glass section I constructed in accordance with the invention and fusion-sealed at its periphery, as indicated at 2, to the rim of a preformed concave glass reflector section 3 to form therewith the envelope of the lamp. The concave inner surface 4 of the reflector section'3 is of approximately paraboloidal shape and is covered with a coating 5 of a suitable metal, such as aluminum for instance, to form a reflecting surface. A concentrated light source or filament 6, preferably in the form of 1a horizontally extending linear coil of tungsten wire, is mounted within the lamp envelope at approximately the focal point of the reflecting surface 5. The filament 6 is electrically connected to and is supported in place within the lamp envelope by lead-in wires 1 which extend through openings (not shown) in the wall of the reflector section 3 at the rear thereof and are secured at their outer ends, as by soldering for instance, to metal thimbles or ferrules 8 fusion-sealed at their rims to the exterior of the reflector section 3 around the said openings. Metal contact or terminal lugs 9 are suitably secured, as by soldering, to the closed ends of the metal thimbles 8 and are provided with terminal screws [0 for connecting the lamp to a suitable source of current supply; The lamp envelope is exhausted and, if desired, filled with a suitable inert gas such as argon, nitrogen, or mixtures thereof, through an exhaust tubulation H at the apex of the reflector section 3, which tubulation is afterwards tipped-off, as indicated at l2, to hermetically seal the lamp envelope. As shown, the cover glass section I may be provided with suitable lightrefracting media l.3. in the form of flutes, prisms. or both, to constitute :a llens for :redirectingithe light rays passing therethrough from the reflector 3 into a beam of the desired form or pattern.

In order to reduce the surface reflection from the lens I and to increase thelliglitiitransniission therethrough, the inner and outer surfaces M and [5 of the lens are providedwithsubstantially transparent reflection-reducing coatings l6 and I1, respectively, similar to that described and claimed in U. S. Patent 2,432;&8*l, -Moulton; lThese coatings i6 and I! comprise a layer :ofdiscrete, sub-microscopic, microgranular, transparent solid particles I 8 (Fig. 2) of colloi'clail'nalture whereby a layer is produced in which the index of lrefraction behaves as if it varied from substantially unity-at the outeror .layer-air-surface thereof :to an index of refraction approaching .that of .the article itself .at the inner or layer-article surface thereof. This is accomplished by. applying on the article I to be treatedasuspensionzcomprising a liquid having dispersed .therein=col1oidal particles which, upon :the evaporation .of the liquid, deposits vsuchta layer :upon the article, which article may beof glass, .plastic-or other material which is .thesubstrate.

Thecoating suspension employed may .comprise any of those describediin.theaabove-mentioned Moulton patent, .but because .of its chemicalinertness and stability inwtheformzof .a sol, a colloidal silica sol .in water, alcohol or other suitable solvents .such .as ethylene .glycol monoethyl ether, or .a mixture of solvents, is-most useful for .this purpose, the silica-beingpresent in the formof sub-microscopic, discrete,-.micro granular, transparent-solid particles.

While the thickness 0f the -:refiection-e-reduoing coatings l6 and I] may be =varied.'over 1a relatively wide range withinzlimits -.of-.the-order of, or an .odd multiple of, one-quarter wavelength of the 'light transmitted, the ccoloraof the reflected light and 1 consequentlyethe -.color .of-ithe transmitted :light is affected by the thickness of the .colloidal layer. lHowevenzit has been found that in general the .mostieificient .layer and \one having the least effect upon the color of the transmitted .light .results from the application :of a layer of such thickness athatthe reflected light is :of a reddish-purple color. For use .on.a ve hicle headlamp, .itis preferableltoxemploy such a vcoating which .presents a distinct--:colored appearance by reflected light :sineesuch alcolored reflection provides :a readily visible indication of the presence .of the coating and lthereiorelserves to distinguish lamps or.lenses provided :with the reflection-reducing coatings from those which lareinot.

In accordance with :theinvention, the-durability .and resistance-of these coatings I5, I?! against removal by weathering, washing, :wiping, etc., is greatly increased to the pOint-fi-Wheredt. becomes practical to use such coatings on the lens surfaces. of vehicIeheadIamps-by incorporating in the coating suspension arelativelysmall amount of a suitable imetallic compoundwvhich 'is solublev in the lalcohol-or other solvent-employed for the suspension and which, lupcnrhea'tingor baking of the coated lens or other glass-article I at a suitable elevated .temperatureihelow the softening point of .the'glass article and tier 9. suit-able length of time, will meactawith the glass of the article and act in the manner of a flux to either cause the silica or other particles of the coating material to stick to the glass article or, if not to stick, to leave impressions in the glass surface which serve to produce the same reflection-reducing effect of the :silica particles themselves. For ordinary type hard glasses having a softening point of around 700-800 C., the heating or baking temperature of the coated .gla'ssarticle may befrom around 500-700 C. or -so,- i. e., -at:atemperature slightly below the temperature at which the glass article would normally soften and cause the silica particles of the coating material to soak into the glass article and. disappear thereinto. The time of heating :or :baking .mayvary according to the nature of .the coating material as well as of the glass ar- "ticle, and according'to the temperature at which the heating or baking operation is carried out. Ordinarily, however, a heating or baking time up tdaroundone-half to one hour or-so will prove sufiicient in most cases. In the case of certain types of :additive materials according to the invention, .the heating or baking of the coated .ar-

e ticle, instead of being performed as a-separate to the invention. As examples of suitable vaddi tivelmaterials .for. the purposes of .theinvention, suitable compoundsofone or moreofthemetals zinc, silver, copper, nickel, iron, titanium, tin, lead, manganese and chromium may be employed, .for. instance, saltssuchas zinc-acetate,-silver nitrate, silver. chloride, silver sulphide,.cupric or cuprous chlorideor sulphide, .iron chloride, tin .chloride, manganeselchloride, titanium tetrachloride, lead nitrate, or other compounds such as chromicoxidefor instance. Of thesematerials, .Ihowever, it is preferred to employ those which, in the small amounts required to-accomplish the objects .of the invention, do not impart a separate color of their own .to the glass article or .materially affect the substantially transparent character .of the coating, which discolor-ing may be objectionableforsome reason orxother. For example, where one of the compounds of .-zinc, silver, titanium, tin or lead specifically enumerated above is employed, the small-amount of the metal present .-in "accordance with the invention does not affect in any way the normal reddishpurple reflection color of 'the reflection-reducing coating per se, whichtcolored reflection serves as ,an indication of the presence of suchcoating article and reduce the transparency-of the; coated article. '-I-'hetamount of-the added'ingredientrnecessary to provide the desired effect in "accordance with the invention maybe determined by trial, but inzany eventit should not be'more than the 7; amount that will dissolve in-=the solvent of the accepts particular coating suspension employed. Ordinarily, the presence in the coating suspension of around one to a few per cent by weight of the added ingredient will give the desired result. The exact proportions, howevenof the materials forming the coating suspension will depend upon the particular mode of application of the coating to the article, it being usually necessary to vary the proportions in order to obtain the desired final coating thickness.

Of the various compounds specifically mentioned above, zinc acetate has been found to be particularly useful because of its distinctive prop-- erty of absorbing into the giass of the article, the manner according to the invention, at relatively low temperatures around 500 C. or so. As a result, where the article to be coated constitutes a part of an electric incandescent lamp or similar device, inlthe manufacture of which glass heating or fusing. cperations are normally required, the heat of. such normal iamp-making processes, e. g., the fusion of a hard glass lens to the reflector section of a sealed beam type lamp, can be utilized for the heating or baking of the coated article in accordance with the invention since the glass lens attains a sufficiently high temperature up to around 500 C. for such purpose. A further advantage in the use of zincacetate as the additive material resides in the fact that where the glass article to which the coating is to be applied is provided with sealing surfaces which are to be fused to another sealin surface, as in the aforementioned case of the fusion of a glass lens. to the glass reflector section of 'a "sealed' beam type lamp, the presence on such sealing surfaces of a coating suspension according to the invention, and having zinc acetate as the additive ingredient, does not prevent the formation of a good fused seal between such sealing surfaces such as occurs in the case where use is made of the colloidal silica solution alone, i. e., without the incorporation therein of zinc acetate or possibly others of the additive ingredients specified hereinabove in accordance with the invention. Therefore, in the application of such a zinc-acetate containing coating to the article, it is unnecessary to remove or Wipe the coating suspension off the sealing surface of the article prior to the sealing operation, and, since the labor cost is the major cost item in the application of a reflection-reducing coating to the article, a very appreciable saving in the cost of such coating application is effected by the elimination of such a wiping operation to clean the sealing surfaces. As an example of a suitable zinc-acetate containing coating suspension according to the invention, the following composition, by weight, has been found to be entirely satisfactory:

Percent Colloidal silica 4.5- 6.5 Zinc acetate 0.5- 3.5 Alcohol or other solvent 90.0-95.0

While the actual manner in which the additive metallic ingredient functions to produce the results according to the invention is not known for certain, it is thought that the zinc, silver or other metal comprising the addition ingredient coats the colloidal silica or other solid particles of the coating suspension with a thin binder layer l9 (Fig. 2) which dissolves or is absorbed into the glass of the article to be coated, as indicated at in Fig. 2, and either pulls the silica or other solid particles along with it into the glass or else replaces the soda in the glass and drives the tatter out to the surface where the soda then acts as a flux to stick the silica or other solid coating particles to the glass. But regardless of the exact manner in which the addition ingredient functions in the process according to the invention, there is no question but that a much more durable and permanent reflection-reducing coating is; produced which withstand weathering and repeated washings and wiping and cleaning without being rubbed off or removed from the glass article.

The coatings. i6 and i1 may be applied to the glass lens or other article i in a variety of ways. Thus, the lens or other article I may be immersed in the coating suspension containing the addition ingredient according to the invention or the coating suspension poured over the surface of the glass article or the said surface washed with the. coating suspension. The article is then immediately spun to remove the excess liquid, or the article to be coated may be spun and, while spinning, a suitable quantity of the coating suspension pouredupon it orotherwise applied. The speedof spinning controls toa certain extent the amount of coating material remaining on the surface and therefore the resultant thickness of the coating. Upon theevaporation of the solvent either case, the colloidal particles [8- remain onthe surfaces F4 and i5 of the article l and it isbelieved that the surfaces their con-- sist of minute projecting irregularities whose spacings are considerably smaller than the wave length of the light, the concentration of the said particles form-ingthe irregularities decreasing from the surface of the article outward where-"- by there is a gradual increase of effective index of refraction from approximate unity at the layer-air interface to a value approaching that of the substrate at the layer-substrate interface. After drying of the coating on the glass article I the latter is then heated either by the heat developed during the processing of the lamp as previously described, or it may be necessary to bake the article in an oven or lehr at elevated temperatures, from approximately 500700 C. or so in the case of zinc acetate, or from approximately 600-700 C. or so in the case of the other additive compounds specifically mentioned hereinabove, for periods up to around one-half to one hour or so, as stated above, to effect the sticking or bonding of the colloidal particles [8 of the coatings I6, I! to the surface of the glass article i in accordance with the invention.

While I have shown and described the preferred embodiment of my invention, it will be obvious to those skilled in the art that various modifications may be made without departing from the invention. Thus, instead of providing both the inner and outer surfaces of the lens I with reflection-reducing coatings, such coatings may be applied to only one or the other of the said surfaces. Likewise, if desired, only the outer coating I! may be of the type according to the invention since the inner coating 16 is not ,exposed and is not subject to removal by weathering, washing, wiping, etc., so that it does not have to be as durable and permanent as the other coating I1.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of forming a transparent reflection-reducing coating on the surface of a hard glass article having a refractive index greater than unity which method comprises, coating the said surface of the article with a suspension comprising transparent colloidal particles of silica dispersed in a liquid dispersion medium having in solution therewith from approximately a half to a few per cent by weight of zinc acetate, removing the excess of coating material from the surface of the article and drying the remaining coating material thereon, and then heating the coated article at a temperature of from about 500-700 C. until the colloidal silica is firmly attached to said surface.

2. The method of forming a transparent refiection-reducing coating on the surface of a hard glass article having a refractive index greater than unity which method comprises, coating the said surface of the article with a suspension consisting essentially, by weight, of approximately from 4.5 to 6.5 per cent of colloidal silica, approximately from 0.5 to 3.5 per cent of zinc acetate, and approximately from 90 to 95 per cent of a solvent for the said silica and zinc acetate, removing the excess of coating material from the surface of the article and drying the remaining coating material thereon, and then heating the coated article at a temperature of from about 500-700 C. until the colloidal silica is firmly attached to said surface.

3. The method of forming a transparent reflection-reducing coating on the surface of a glass article having a refractive index greater than unity which method comprises, coating the said surface of the article with a suspension comprising transparent colloidal silica particles dispersed in a liquid dispersion medium having in solution therewith from approximately a half to a few per cent by weight of a metallic salt decomposable into free metal at a temperature below the softening temperature of the glass article, the said metal being absorbable into the glass of the article when the latter is in a heated condition at a temperature just below its softening point, and then heating the coated article at a temperature of from about 500 to 700 C., which is above the decomposition temperature of the said metallic salt but below the softening temperature of the glass article for a sufllcient time to decompose the said metallic salt into free metal and cause absorption of the latter into the said surface of the glass article at the areas where the silica particles contact the surface of the glass whereby to attach the said colloidal silica particles to said surface.

4. The method as set forth in claim 3 wherein the said metallic salt is a zinc salt.

5. The method as set forth in claim 3 wherein the said metallic salt is a silver salt.

6. The article prepared by the method of claim 3.

'7. The article prepared by the method of claim 4.

8. The article prepared by the method of claim 5.

STUART McLEAN.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,123,957 Orth July 19, 1938 2,344,250 Jones Mar. 14, 1944 2,432,484 Moulton Dec. 9, 1947 

3. THE METHOD FORMING A TRANSPARENT REFLECTION-REDUCING COATING ON THE SURFACE OF A GLASS ARTICLE HAVING A REFRACTIVE INDEX GREATER THAN UNITY WHICH METHOD COMPRISES, COATING THE SAID SURFACE OF THE ARTICLE WITH A SUSPENSION COMPRISING TRANSPARENT COLLOIDAL SILICA PARTICLES DISPERSED IN A LIQUID DISPERSION MEDIUM HAVING IN SOLUTION THEREWITH FROM APPROXIMATELY A HALF TO A FEW PER CENT BY WEIGHT OF A METALLIC SALT DECOMPOSABLE INTO FREE METAL AT A TEMPERATURE BELOW THE SOFTENING TEMPERATURE OF THE GLASS ARTICLE, THE SAID METAL BEING ABSORBABLE INTO THE GLASS OF THE ARTICLE WHEN THE LATTER IS IN A HEATED CONDITION AT A TEMPERATURE JUST BELOW ITS SOFTENING POINT, AND THEN HEATING THE COATED ARTICLE AT A TEMPERATURE OF FROM ABOUT 500* TO 700* C., WHICH IS ABOVE THE DECOMPOSITION TEMPERATURE OF THE SAID METALLIC SALT BUT BELOW THE SOFTENING TEMPERATURE OF THE GLASS ARTICLE FOR A SUFFICIENT TIME TO DECOMPOSE THE SAID METALLIC SALT INTO FREE METAL AND CAUSE ABSORPTION OF THE LATTER INTO THE SAID SURFACE OF THE GLASS ARTICLE AT THE AREAS WHERE THE SILICA PARTICLES CONTACT THE SURFACE OF THE GLASS WHEREBY TO ATTACH THE SAID COLLOIDAL SILICA PARTICLES TO SAID SURFACE. 